Re: [PATCH] mm: shmem: convert to use folio_zero_range()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/10/25 20:21, Huang, Ying wrote:
> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
> 
>> On 2024/10/25 15:47, Huang, Ying wrote:
>>> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
>>>
>>>> On 2024/10/25 10:59, Huang, Ying wrote:
>>>>> Hi, Kefeng,
>>>>> Kefeng Wang <wangkefeng.wang@huawei.com> writes:
>>>>>
>>>>>> +CC Huang Ying,
>>>>>>
>>>>>> On 2024/10/23 6:56, Barry Song wrote:
>>>>>>> On Wed, Oct 23, 2024 at 4:10 AM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>> ...
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> On 2024/10/17 23:09, Matthew Wilcox wrote:
>>>>>>>>>>>>>>>>>>>>>>> On Thu, Oct 17, 2024 at 10:25:04PM +0800, Kefeng Wang wrote:
>>>>>>>>>>>>>>>>>>>>>>>> Directly use folio_zero_range() to cleanup code.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> Are you sure there's no performance regression introduced by this?
>>>>>>>>>>>>>>>>>>>>>>> clear_highpage() is often optimised in ways that we can't optimise for
>>>>>>>>>>>>>>>>>>>>>>> a plain memset().  On the other hand, if the folio is large, maybe a
>>>>>>>>>>>>>>>>>>>>>>> modern CPU will be able to do better than clear-one-page-at-a-time.
>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Right, I missing this, clear_page might be better than memset, I change
>>>>>>>>>>>>>>>>>>>>>> this one when look at the shmem_writepage(), which already convert to
>>>>>>>>>>>>>>>>>>>>>> use folio_zero_range() from clear_highpage(), also I grep
>>>>>>>>>>>>>>>>>>>>>> folio_zero_range(), there are some other to use folio_zero_range().
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:           folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>>>>>> folio_size(folio));
>>>>>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
>>>>>>>>>>>>>>>>>>>>>> 0, folio_size(f));
>>>>>>>>>>>>>>>>>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
>>>>>>>>>>>>>>>>>>>>>> 0, folio_size(f));
>>>>>>>>>>>>>>>>>>>>>> fs/libfs.c:     folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>>>>>> fs/ntfs3/frecord.c:             folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>>>>>> folio_size(folio));
>>>>>>>>>>>>>>>>>>>>>> mm/page_io.c:   folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>>>>>> mm/shmem.c:             folio_zero_range(folio, 0, folio_size(folio));
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> IOW, what performance testing have you done with this patch?
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> No performance test before, but I write a testcase,
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> 1) allocate N large folios (folio_alloc(PMD_ORDER))
>>>>>>>>>>>>>>>>>>>>>> 2) then calculate the diff(us) when clear all N folios
>>>>>>>>>>>>>>>>>>>>>>               clear_highpage/folio_zero_range/folio_zero_user
>>>>>>>>>>>>>>>>>>>>>> 3) release N folios
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> the result(run 5 times) shown below on my machine,
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> N=1,
>>>>>>>>>>>>>>>>>>>>>>                   clear_highpage  folio_zero_range    folio_zero_user
>>>>>>>>>>>>>>>>>>>>>>              1      69                   74                 177
>>>>>>>>>>>>>>>>>>>>>>              2      57                   62                 168
>>>>>>>>>>>>>>>>>>>>>>              3      54                   58                 234
>>>>>>>>>>>>>>>>>>>>>>              4      54                   58                 157
>>>>>>>>>>>>>>>>>>>>>>              5      56                   62                 148
>>>>>>>>>>>>>>>>>>>>>> avg       58                   62.8               176.8
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> N=100
>>>>>>>>>>>>>>>>>>>>>>                   clear_highpage  folio_zero_range    folio_zero_user
>>>>>>>>>>>>>>>>>>>>>>              1    11015                 11309               32833
>>>>>>>>>>>>>>>>>>>>>>              2    10385                 11110               49751
>>>>>>>>>>>>>>>>>>>>>>              3    10369                 11056               33095
>>>>>>>>>>>>>>>>>>>>>>              4    10332                 11017               33106
>>>>>>>>>>>>>>>>>>>>>>              5    10483                 11000               49032
>>>>>>>>>>>>>>>>>>>>>> avg     10516.8               11098.4             39563.4
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> N=512
>>>>>>>>>>>>>>>>>>>>>>                   clear_highpage  folio_zero_range   folio_zero_user
>>>>>>>>>>>>>>>>>>>>>>              1    55560                 60055              156876
>>>>>>>>>>>>>>>>>>>>>>              2    55485                 60024              157132
>>>>>>>>>>>>>>>>>>>>>>              3    55474                 60129              156658
>>>>>>>>>>>>>>>>>>>>>>              4    55555                 59867              157259
>>>>>>>>>>>>>>>>>>>>>>              5    55528                 59932              157108
>>>>>>>>>>>>>>>>>>>>>> avg     55520.4               60001.4            157006.6
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> folio_zero_user with many cond_resched(), so time fluctuates a lot,
>>>>>>>>>>>>>>>>>>>>>> clear_highpage is better folio_zero_range as you said.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Maybe add a new helper to convert all folio_zero_range(folio, 0,
>>>>>>>>>>>>>>>>>>>>>> folio_size(folio))
>>>>>>>>>>>>>>>>>>>>>> to use clear_highpage + flush_dcache_folio?
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> If this also improves performance for other existing callers of
>>>>>>>>>>>>>>>>>>>>> folio_zero_range(), then that's a positive outcome.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> ...
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> hi Kefeng,
>>>>>>>>>>>>>>>>>> what's your point? providing a helper like clear_highfolio() or similar?
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Yes, from above test, using clear_highpage/flush_dcache_folio is better
>>>>>>>>>>>>>>>>> than using folio_zero_range() for folio zero(especially for large
>>>>>>>>>>>>>>>>> folio), so I'd like to add a new helper, maybe name it folio_zero()
>>>>>>>>>>>>>>>>> since it zero the whole folio.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> we already have a helper like folio_zero_user()?
>>>>>>>>>>>>>>>> it is not good enough?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Since it is with many cond_resched(), the performance is worst...
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Not exactly? It should have zero cost for a preemptible kernel.
>>>>>>>>>>>>>> For a non-preemptible kernel, it helps avoid clearing the folio
>>>>>>>>>>>>>> from occupying the CPU and starving other processes, right?
>>>>>>>>>>>>>
>>>>>>>>>>>>> --- a/mm/shmem.c
>>>>>>>>>>>>> +++ b/mm/shmem.c
>>>>>>>>>>>>>
>>>>>>>>>>>>> @@ -2393,10 +2393,7 @@ static int shmem_get_folio_gfp(struct inode
>>>>>>>>>>>>> *inode, pgoff_t index,
>>>>>>>>>>>>>                 * it now, lest undo on failure cancel our earlier guarantee.
>>>>>>>>>>>>>                 */
>>>>>>>>>>>>>
>>>>>>>>>>>>>                if (sgp != SGP_WRITE && !folio_test_uptodate(folio)) {
>>>>>>>>>>>>> -               long i, n = folio_nr_pages(folio);
>>>>>>>>>>>>> -
>>>>>>>>>>>>> -               for (i = 0; i < n; i++)
>>>>>>>>>>>>> -                       clear_highpage(folio_page(folio, i));
>>>>>>>>>>>>> +               folio_zero_user(folio, vmf->address);
>>>>>>>>>>>>>                        flush_dcache_folio(folio);
>>>>>>>>>>>>>                        folio_mark_uptodate(folio);
>>>>>>>>>>>>>                }
>>>>>>>>>>>>>
>>>>>>>>>>>>> Do we perform better or worse with the following?
>>>>>>>>>>>>
>>>>>>>>>>>> Here is for SGP_FALLOC, vmf = NULL, we could use folio_zero_user(folio,
>>>>>>>>>>>> 0), I think the performance is worse, will retest once I can access
>>>>>>>>>>>> hardware.
>>>>>>>>>>>
>>>>>>>>>>> Perhaps, since the current code uses clear_hugepage(). Does using
>>>>>>>>>>> index << PAGE_SHIFT as the addr_hint offer any benefit?
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> when use folio_zero_user(), the performance is vary bad with above
>>>>>>>>>> fallocate test(mount huge=always),
>>>>>>>>>>
>>>>>>>>>>             folio_zero_range   clear_highpage         folio_zero_user
>>>>>>>>>> real    0m1.214s             0m1.111s              0m3.159s
>>>>>>>>>> user    0m0.000s             0m0.000s              0m0.000s
>>>>>>>>>> sys     0m1.210s             0m1.109s              0m3.152s
>>>>>>>>>>
>>>>>>>>>> I tried with addr_hint = 0/index << PAGE_SHIFT, no obvious different.
>>>>>>>>>
>>>>>>>>> Interesting. Does your kernel have preemption disabled or
>>>>>>>>> preemption_debug enabled?
>>>>>>>>
>>>>>>>> ARM64 server, CONFIG_PREEMPT_NONE=y
>>>>>>> this explains why the performance is much worse.
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> If not, it makes me wonder whether folio_zero_user() in
>>>>>>>>> alloc_anon_folio() is actually improving performance as expected,
>>>>>>>>> compared to the simpler folio_zero() you plan to implement. :-)
>>>>>>>>
>>>>>>>> Yes, maybe, the folio_zero_user(was clear_huge_page) is from
>>>>>>>> 47ad8475c000 ("thp: clear_copy_huge_page"), so original clear_huge_page
>>>>>>>> is used in HugeTLB, clear PUD size maybe spend many time, but for PMD or
>>>>>>>> other size of large folio, cond_resched is not necessary since we
>>>>>>>> already have some folio_zero_range() to clear large folio, and no issue
>>>>>>>> was reported.
>>>>>>> probably worth an optimization. calling cond_resched() for each page
>>>>>>> seems too aggressive and useless.
>>>>>>
>>>>>> After some test, I think the cond_resched() is not the root cause,
>>>>>> no performance gained with batched cond_resched(), even I kill
>>>>>> cond_resched() from process_huge_page, no improvement.
>>>>>>
>>>>>> But when I unconditionally use clear_gigantic_page() in
>>>>>> folio_zero_user(patched), there is big improvement with above
>>>>>> fallocate on tmpfs(mount huge=always), also I test some other testcase,
>>>>>>
>>>>>>
>>>>>> 1) case-anon-w-seq-mt: (2M PMD THP)
>>>>>>
>>>>>> base:
>>>>>> real    0m2.490s    0m2.254s    0m2.272s
>>>>>> user    1m59.980s   2m23.431s   2m18.739s
>>>>>> sys     1m3.675s    1m15.462s   1m15.030s	
>>>>>>
>>>>>> patched:
>>>>>> real    0m2.234s    0m2.225s    0m2.159s
>>>>>> user    2m56.105s   2m57.117s   3m0.489s
>>>>>> sys     0m17.064s   0m17.564s   0m16.150s
>>>>>>
>>>>>> Patched kernel win on sys and bad in user, but real is almost same,
>>>>>> maybe a little better than base.
>>>>> We can find user time difference.  That means the original cache hot
>>>>> behavior still applies on your system.
>>>>> However, it appears that the performance to clear page from end to
>>>>> begin
>>>>> is really bad on your system.
>>>>> So, I suggest to revise the current implementation to use sequential
>>>>> clearing as much as possible.
>>>>>
>>>>
>>>> I test case-anon-cow-seq-hugetlb for copy_user_large_folio()
>>>>
>>>> base:
>>>> real    0m6.259s    0m6.197s    0m6.316s
>>>> user    1m31.176s   1m27.195s   1m29.594s
>>>> sys     7m44.199s   7m51.490s   8m21.149s
>>>>
>>>> patched(use copy_user_gigantic_page for 2M hugetlb too)
>>>> real    0m3.182s    0m3.002s    0m2.963s
>>>> user    1m19.456s   1m3.107s    1m6.447s
>>>> sys     2m59.222s   3m10.899s   3m1.027s
>>>>
>>>> and sequential copy is better than the current implementation,
>>>> so I will use sequential clear and copy.
>>> Sorry, it appears that you misunderstanding my suggestion.  I
>>> suggest to
>>> revise process_huge_page() to use more sequential memory clearing and
>>> copying to improve its performance on your platform.
>>> --
>>> Best Regards,
>>> Huang, Ying
>>>
>>>>>> 2) case-anon-w-seq-hugetlb:(2M PMD HugeTLB)
>>>>>>
>>>>>> base:
>>>>>> real    0m5.175s    0m5.117s    0m4.856s
>>>>>> user    5m15.943s   5m7.567s    4m29.273s
>>>>>> sys     2m38.503s   2m21.949s   2m21.252s
>>>>>>
>>>>>> patched:
>>>>>> real    0m4.966s    0m4.841s    0m4.561s
>>>>>> user    6m30.123s   6m9.516s    5m49.733s
>>>>>> sys     0m58.503s   0m47.847s   0m46.785s
>>>>>>
>>>>>>
>>>>>> This case is similar to the case1.
>>>>>>
>>>>>> 3) fallocate hugetlb 20G (2M PMD HugeTLB)
>>>>>>
>>>>>> base:
>>>>>> real    0m3.016s    0m3.019s    0m3.018s
>>>>>> user    0m0.000s    0m0.000s    0m0.000s
>>>>>> sys     0m3.009s    0m3.012s    0m3.010s
>>>>>>
>>>>>> patched:
>>>>>>
>>>>>> real    0m1.136s    0m1.136s    0m1.136s
>>>>>> user    0m0.000s    0m0.000s    0m0.004s
>>>>>> sys     0m1.133s    0m1.133s    0m1.129s
>>>>>>
>>>>>>
>>>>>> There is big win on patched kernel, and it is similar to above tmpfs
>>>>>> test, so maybe we could revert the commit c79b57e462b5 ("mm: hugetlb:
>>>>>> clear target sub-page last when clearing huge page").
>>
>> I tried the following changes,
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 66cf855dee3f..e5cc75adfa10 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6777,7 +6777,7 @@ static inline int process_huge_page(
>>                  base = 0;
>>                  l = n;
>>                  /* Process subpages at the end of huge page */
>> -               for (i = nr_pages - 1; i >= 2 * n; i--) {
>> +               for (i = 2 * n; i < nr_pages; i++) {
>>                          cond_resched();
>>                          ret = process_subpage(addr + i * PAGE_SIZE, i,
>>                          arg);
>>                          if (ret)
>>
>> Since n = 0, so the copying is from start to end now, but not
>> improvement for case-anon-cow-seq-hugetlb,
>>
>> and if use copy_user_gigantic_pager, the time reduced from 6s to 3s
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index fe21bd3beff5..2c6532d21d84 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6876,10 +6876,7 @@ int copy_user_large_folio(struct folio *dst,
>> struct folio *src,
>>                  .vma = vma,
>>          };
>>
>> -       if (unlikely(nr_pages > MAX_ORDER_NR_PAGES))
>> -               return copy_user_gigantic_page(dst, src, addr_hint,
>>                  vma, nr_pages);
>> -
>> -       return process_huge_page(addr_hint, nr_pages, copy_subpage, &arg);
>> +       return copy_user_gigantic_page(dst, src, addr_hint, vma, nr_pages);
>>   }
> 
> It appears that we have code generation issue here.  Can you check it?
> Whether code is inlined in the same way?
> 

No different, and I checked the asm, both process_huge_page and 
copy_user_gigantic_page are inlined, it is strange...

> Maybe we can start with
> 
> modified   mm/memory.c
> @@ -6714,7 +6714,7 @@ EXPORT_SYMBOL(__might_fault);
>    * operation.  The target subpage will be processed last to keep its
>    * cache lines hot.
>    */
> -static inline int process_huge_page(
> +static __always_inline int process_huge_page(
>   	unsigned long addr_hint, unsigned int nr_pages,
>   	int (*process_subpage)(unsigned long addr, int idx, void *arg),
>   	void *arg)
> 
> --
> Best Regards,
> Huang, Ying
>